This invention is directed to a system and method for performing color corrections on color image data. More particularly, this invention is directed to a system and method for setting output characteristics of an image rendering device. This invention is particularly useful in improving color quality and color fidelity with regards to color saturation, contrast and brightness on data that is to be output on an image processing apparatus.
The color output of an electronic printer, copier, or other document processing device requires pre-adjustment based upon calibration/characterization data. The calibration/characterization data is used to improve the output quality and color fidelity with regards to color saturation, contrast and brightness.
Saturation is the intensity of a color, from neutral gray through pastel to saturated color, in a given color space model. Color saturation control is a process by which the amount of color in an image is increased or decreased without affecting the contrast of the image. For example, when saturation is lowered, the amount of white in an image is increased, giving the image a washed-out appearance. Thus, adjusting the color saturation can result in an image that is black-and-white or has very strong colors. Adjustment of the saturation of an image is typically accomplished by converting the Red-Green-Blue, or RGB color values to a luminance-chrominance color space. Once converted, the saturation is increased within the luminance-chrominance color space. The saturated image is then converted back to RGB color values reflecting the adjusted saturation levels. This technique, however, can result in a saturated image lacking the proper hue. For example, the hue, or saturation relationship in all colors, may get too red. Earlier attempts to compensate for the improper hue included the use of a different color space, i.e., not luminance-chrominance, using masking equations in density space for a default printer setup, or using multi-dimensional lookup tables. These methods, however, fail to adequately provide ease of user adjustment to saturation levels.
Contrast is the rate of change of the gray values of the intensity of color, brightness, or shading of an area occupied by a display element, group or image and the intensity of an area not occupied by a display element, group or image. Stated another way, contrast is the range of optical density and tone on a photographic negative or print (or the extent to which adjacent areas on a television screen differ in brightness). Adjustment of contrast settings by a user has also met with difficulties. The traditional contrast adjustment was accomplished using a gamma function implemented as a lookup table or as a mathematical exponential. For performance reasons, the lookup table is preferable, however a lookup table is an exceedingly awkward technique for contrast adjustment by a user.
Brightness is an attribute of visual perception in which a source appears to emit a given amount of light. Brightness adjustment, as opposed to contrast and saturation adjustment, is suitably accomplished using a multiplicative constant applied in RGB or Cyan-Magenta-Yellow color. As previously noted, adjustment of saturation, contrast and brightness by a user when using the aforementioned techniques is awkward, ineffective, or both.
By way of background, a color space is typically a particular model of colors, represented by a tri-dimensional map. For each of three broad classes of color related devices, a series of base algorithmic models are described that perform a transformation between color spaces. The models provide a range of color quality and performance results equating to different trade-offs in memory footprint, performance and image quality. The device device-dependent and device-independent color spaces are typically divided into three basic families, CIEXYZ, RGB and Cyan-Magenta-Yellow, or CMY. It should be noted that a subset of the CIEXYZ spaces are also defined as connection spaces.
The CIELAB color space gets its name from a color space that uses three values to describe the precise three-dimensional location of a color inside a visible color space. “CIE”, or Commission Internationale de l-Eclairages, is an international body of color scientists whose standards make it possible to communicate color information accurately “L” describes relative lightness, “A” represents relative redness-greenness, and “B” represents relative yellowness-blueness. The CIEXYZ structure contains the X, Y, and Z coordinates of a specific color in a specified color space.
A color model that describes each color in terms of the quantity of each secondary color, cyan, magenta, yellow, and black is commonly referred to as the CMYK color model. The CMYK system is used for printing. For mixing of pigments, it is better to use the secondary colors, since they mix subtractively instead of additively. The secondary colors of light, cyan, magenta and yellow, correspond to the primary colors of pigment (blue, red and yellow). In addition, although black could be obtained by mixing these three in equal proportions, in four-color printing, black is typically used as a separate colorant. The K in CMYK stands for ‘Key’ or ‘blacK,’ so as not to cause confusion with the B in RGB (Red, Green, Blue) color model.
As will be known in the art, the difference between the two color models is based on how color is produced. Red, green, and blue are projected light and cyan, magenta, yellow, and black are reflected light. Combining varying amounts of red, green and blue will faithfully create the spectrum of colors. By varying the amounts of red, green and blue, an image may be projected onto the phosphors of a monitor or television screen. However, pigments added to paper, successively, will darken the paper. Using selective light colors will recreate the spectrum on white paper. Thus, cyan will absorb red, magenta will absorb green and yellow will absorb blue. These are called the subtractive primary colors, or secondary colors. Because of the lightness of cyan, magenta and yellow, black, is used for the gray components.
Often the color conversion from one color space to a second color space is not flawless and there are errors in the quality, intensity, lightness, or other features of the second color-space. For example, the CMYK printing materials for use in an image processing apparatus are not perfectly complementary colors of RGB, and will often generate solid blue colors that contain too much purple color.
Thus, there exists a need for a method and system for a user to adjust saturation, contrast and brightness without detrimentally affecting other portions of the image. There exists a need for a method and system of saturation, contrast and brightness adjustment by a user that is independent of the calibration of a document processing device.
The subject invention remedies the aforementioned problems and provides a method for a user to easily adjust the saturation, contrast and brightness of an image.